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Fe-doped Graphene Nanosheet as an Adsorption Platform of Harmful Gas
Molecules (CO, CO2, SO2 and H2S), and the co-adsorption in O2 environments
Article in Applied Surface Science · January 2018
DOI: 10.1016/j.apsusc.2017.08.216
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1
Fe-doped Graphene Nanosheet as an
Adsorption Platform of Harmful Gas
Molecules (CO, CO2, SO2 and H2S), and
the co-adsorption in O2 environments
Diego Cortés-Arriagada1,*
, Nery Villegas-Escobar2, Daniela E. Ortega
2
1Programa Institucional de Fomento a la Investigación, Desarrollo e Innovación.
Universidad Tecnológica Metropolitana. Ignacio Valdivieso 2409, P.O. Box 8940577, San
Joaquín, Santiago, Chile. *E-mail address: [email protected]
2Laboratorio de Química Teórica Computacional, Facultad de Química, Pontificia
Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Macul, Santiago, Chile.
Abstract. The adsorption of pollutant gases (CO, CO2, SO2 and H2S) onto Fe-doped
graphene nanosheets (FeG) is studied on the basis of density functional theory calculations
at the PBE/Def2-SVP level of theory. The most stable adsorption configurations, binding
characteristics, electronic properties and stability at room temperature of the FeGGas
interactions is fully analyzed. The gas molecules are chemisorbed onto FeG with adsorption
energies in the range of 0.54 to 1.8 eV, with an enhanced adsorption strength compared to
intrinsic graphene. The stability of the FeG-Gas interactions is dominated by Lewis-acid-
base interactions, and its strength is sorted as SO2>CO>H2S>CO2. The adsorption stability
2
is also retained at room temperature (300 K). Due to the strong interaction of SO2, CO, and
H2S, FeG could catalyze or activate these gas molecules, suggesting the possibility of FeG
as a catalyst substrate. The electron acceptor/donor character of CO, CO2, SO2 and H2S
molecules when adsorbed onto FeG causes charge transfer processes that are responsible
for the change in conductance of FeG; thus, the response of the HOMO-LUMO gap of FeG
under gas adsorption could be useful for sensing applications. Furthermore, the analysis of
the co-adsorption in O2 environments shows that the CO2 interaction turns unstable onto
FeG, while the sensing response towards H2S is suppressed. Finally, these results give new
insights into the emerging applications of Fe-doped graphene in gas capture/filtration
devices, solid-state gas sensors or as a catalyst substrate.
Keywords: Fe-doped graphene; adsorption; gas pollutants; DFT calculations; co-
adsorption; gas sensors
1. Introduction
The emission of gaseous pollutants in the atmosphere, industrial and house
environments are of a great concern due to the risk that these pollutants exert[1]. Although
the natural production of most atmospheric pollutants is much higher than artificial and
industrial production, the problem of the latter is that it usually occurs in a much localized
way, so that in places close to the emission source concentrations can be very high[2].
Carbon dioxide (CO2) capture is of interest due to its environmental and economic
relevance. Likewise, carbon monoxide (CO) and hydrogen sulfide (H2S) are considered as
suffocating agents for humans, even in reduced concentrations; as well as sulfur oxides
(SOX), which has a negative contribution to the environment and human health[1, 2].
3
These gases are also recognized to contribute to the global warming through the greenhouse
effect, cause acid rain and photochemical smog, and several respiratory diseases due to its
chronic exposure[3-8]. Therefore the sensing of these gases to avoid chronic exposure is of
a great interest, in addition to the development of materials allowing its collection and
capture before the release of burning or industrial gases to the environment.
An alternative of environmental monitoring to this problem is the emerging
application of graphene as gas sensing or capture material due to its low cost, low power
consumption and high surface area[9, 10]. Graphene is highly stable, causes low
contamination and bind gas molecules by intermolecular interactions to its surface,
reaching high molecular adsorption and storage[9-13]. For instance, the adsorption abilities
of graphene towards toxic gaseous species (such as CO2, CO, NO2, NH3) have been well
described[9, 11, 14-19]. In this regard, most of the recent developments are focused on
materials enhancing the adsorption stability of adsorbates onto graphene. In this sense, the
local reactivity of graphene can be tailored via doping, which creates more reactive
adsorption sites[20-26]. Metals such as Cu, Ag, Au, Ti, Cr, Mn and Pd have been
theoretically considered as dopants in graphene to enhance its adsorption, storage capacity
and sensing properties towards CO, CO2, NO2, NO, H2S and other harmful molecules[27-
31]. Moreover, doping of graphene oxide has been also reported to form stable and
excellent sorbents for gas collection and filtration, even with low interference of O2
molecules[32]. Thus, the application of metal-doped graphene for gas collection/sensing
applications is expected to emerge as these materials will be experimentally available.
Theoretical studies based on the Density Functional Theory (DFT) framework have
shown the excellent sorption properties of Fe-embedded graphene (FeG) for a wide class of
4
air and water pollutants, significantly enhancing the adsorption with respect to intrinsic
graphene through strong Lewis-acid-base interactions[25, 26, 33-36]. Among the
advantages of iron as dopant are its low cost, low environmental toxicity compared to noble
metals, and its high acceptor character, making it an excellent candidate in terms of
improving the sensitivity of graphene at environmental levels. In addition, iron is bonded to
graphene with high binding energies (7.0 eV) and high diffusion barriers (6.8 eV)[37,
38], then forming high stable adsorbents. In this sense, synthesized FeG through aberration-
corrected transmission electron microscopy technique shows high stability at the air and
resistance to oxidants and corrosive species, where the dopants are able to disperse and
bind to defective graphene with low cluster formation[39]. Additionally, the bandgap of
graphene is opened by Fe-doping, which turns it useful for sensing applications[25]; for
example, the CO2 detection onto FeG nanoribbons has been proved[40, 41]. Furthermore,
we recently have theoretically studied the adsorption and sensing properties of FeG toward
nitrogen oxides and formaldehyde[25, 26], indicating that FeG is highly sensitive to these
gas molecules even in the presence of oxygen.
Taking into consideration that FeG emerges as a promising material for adsorption,
filtration, collection and/or sensing of harmful gas molecules, a DFT study was performed
to study the gas adsorption of harmful gas molecules (CO, CO2, SO2 and H2S) as target
gases onto FeG nanosheets, characterizing also the role of O2 interference in the adsorption
mechanism. The FeG-Gas systems were characterized from its geometrical, energetic,
electronic and binding properties. Molecular dynamics studies were performed to analyze
the FeG-Gas interaction stability at ambient conditions, and the adsorption stability was
also characterized in aerobic conditions. As a reference, the gas adsorption was also studied
5
onto pristine graphene. Through this study, FeG is suggested to enhance the gas adsorption
process of toxic gaseous pollutants with negative effects on human health and on the
environment.
2. Computational Methodology
All the calculations were performed at the DFT level in the ORCA 3.0.3
program[42]. The PBE functional[43] was implemented in combination with the all-
electron Def2-SVP basis sets[44-46]. The PBE method was selected due to its wide use in
sorption studies related to the adsorption of small gas molecules onto doped graphene[29,
33, 47-52]. Dispersion corrections for energies and gradients were obtained through the
pair-wise DFT-D3 method, including also the BeckeJohnson damping function to avoid
repulsive interatomic forces at short distances[53, 54]. The dispersion correction (Edisp) is
added to the SCF-PBE energies (ESCFPBE), and the corrected total energies are expressed as
a sum of electronic and dispersion contributions: EPBED3=ESCFPBE+Edisp. Vibrational
frequency calculations were also performed for all the molecular systems; only positive
frequencies were associated with all the vibrational modes in all the systems, indicating that
they correspond to stable energy minimums. The binding nature was analyzed through the
Natural Bond Orbital (NBO) method in the NBO 6.0 program[55]. Wavefunction analyses
were also performed in Multiwfn[56]. The stability of the adsorption configurations was
analyzed in terms of their adsorption energies (Eads):
vdWadsorbateFeGadsorbateFeGads EEEEE (1)
where, EFeG, Eadsorbate and EFeG-adsorbate are the total energies of FeG, adsorbate and FeG-
adsorbate systems, respectively; hence, the more positive values of Eads, the more stable the
6
FeG-adsorbate systems are. EvdW is the dispersion contribution to Eads, which is obtained as
EvdW=Edisp(adsorbent)+Edisp(adsorbate)Edisp(adsorbentadsorbate), where Edisp(i) are the DFT-D3
dispersion corrections of the isolated fragments and the FeG-adsorbate system. Thus,
adsorption energies are decomposed into the sum of electronic and dispersion contributions.
Basis set superposition errors were corrected with the geometrical counterpoise method
gCP[57].
Ab-initio molecular dynamic calculations were performed in order to insure for the
stability of the FeG-gas interactions at room temperature (300 K). Trajectories were
obtained with the ADMP (atom density matrix propagation) method[58-60] via the Verlet
velocity algorithm[61], which include propagation of both the nuclear centers and electron
density, thus, giving an adequate behavior of the chemical bonding under kinetic energy.
This tool has allowed to us analyze the adsorption and chemical stability of pollutants onto
graphene based materials at room conditions[25, 26, 62, 63]. ADMP calculations were
performed in the Gaussian09 program[64], using as input the optimized structures in the
ORCA program. The potential was determined "on-the-fly" at the PBE/6-31G(d) level of
theory with a time step (t) of 0.1 fs. A total time 2.0 ps was used for statistic analyzes (this
is 20000 conformations). Temperature (T) was set to 300 K and controlled by velocity
scaling.
Graphene nanosheets are modeled as a finite carbon cluster (G: C94H24), where the
dangling bonds are saturated with hydrogen atoms. FeG was built by replacing one carbon
atom in the graphene model (FeG: FeC93H24). The dopant concentration in FeG was
retained below 6% to be in agreement with synthesized graphene nanosheets as those with
Si, N and B[65-69]. Note that the selection of these surface models was based on well
7
converged adsorption energies as the surface size increases. Finally, FeG was modeled as a
closed-shell system in its ground state as obtained from previous DFT studies, where higher
spin states are at least 0.2 eV above the closed-shell ground state[37, 38, 63].
3. Results and discussion
3.1 Adsorption energies and geometry
The most stable adsorption configurations of CO, CO2, H2S and SO2 onto pristine
and Fe-doped graphene are displayed in Fig. 1. The adsorption energies and contribution of
dispersion forces are in Table 1.
Table 1. Adsorption energies (Eads) and contribution of dispersion forces (EvdW) of the G-
adsorbate and FeG-adsorbate systems. Energies are in eV.
system Eads EvdW
with pristine graphene (G)
G-CO 0.08 0.15
G-CO2 0.11 0.19
G-H2S 0.15 0.19
G-SO2 0.28 0.29
with Fe-doped graphene (FeG)
FeG-CO 1.60 0.12
FeG-CO2 0.54 0.20
FeG-H2S 1.19 0.18
FeG-SO2 1.80 0.26
Firstly, the interaction of the gas molecules onto intrinsic graphene was studied for
comparison purposes. Fig. 1b shows that all the gas molecules are adsorbed onto pristine
graphene at distances of 3.0-3.2 Å, and with low adsorption energies ranging from 0.08 to
8
0.28 eV (Table 1). The interaction in these cases is entirely due to dispersion interactions as
noted from the EvdW values, where the electronic contribution is repulsive (Eads<EvdW).
Consequently, the interaction is also accompanied by a low electron transfer of the order of
±110-2
|e|. The low adsorption energies indicate that adsorption of CO, CO2, H2S and SO2
onto intrinsic graphene is lowly stable at room temperature. At next, it is studied the
interaction of the gas molecules onto FeG. The attention was focused in the most stable
adsorption configurations (Fig. 1c). All the FeG-Gas interactions take place by chemical
bonding between the gas molecule and dopant atom, with adsorption energies ranging from
0.54 to 1.80 eV. Dispersion energies only contribute up to 0.26 eV to the adsorption
energies. With respect to intrinsic graphene, the adsorption energies increase in 1.52, 0.43,
1.04 and 1.52 eV CO, CO2, H2S and SO2, respectively, when FeG is implemented as a gas
adsorbent; thus, the adsorption strength is enhanced in at least 390% compared to graphene.
Fig. 1. Optimized molecular structures of: a) the isolated gas molecules; b) gas molecules
adsorbed onto pristine graphene (G); and (c) gas molecules adsorbed onto FeG. Distances
are in angstroms (Å) and angles in degrees (°). Color code: White (H); grey (C); red (O);
yellow (S); and orange (Fe).
9
In the case of the FeG-CO system, CO acquires an almost perpendicular adsorption
configuration onto FeG by means of C-O bonding (C-end configuration) that is similar as
obtained onto Al, Co, Cu, Ag and Au-modified graphene[20, 29, 47, 70]. This adsorption
configuration is explained because of CO has a single 2p lone pair orbital according to its
Lewis structure, which is parallel with respect to the C-O bonding. The C-end configuration
has an adsorption energy of 1.60 eV and intermolecular Fe-C bond length of dFe-C=1.88 Å.
Due to the interaction, the C-O bond distance is slightly elongated from 1.14 to 1.17 Å with
respect to the free CO molecule (Fig. 1a). Note that the adsorption configuration in the O-
end mode was found to be 1 eV less stable that the C-end configuration. The adsorption
energy of the FeG-CO system is comparable with those reported of 1.38 and 1.45 eV in the
same C-end configuration[38, 71]; note that these comparisons are on the basis of DFT
calculations with the PW91 and PBE functionals without dispersion corrections, explaining
the low differences in stability. On the other hand, the FeG-CO2 system has an low Eads
value of 0.54 eV, which is 1 eV lower compared to the CO adsorption; the latter indicates
that CO is preferably adsorbed than CO2. In this case, CO2 is absorbed by Fe-C and Fe-O
bonds of 2.16 and 2.00 Å, respectively. Additionally, the O-C-O angle is decreased from
180° to 154°, while the interacting C-O bond of the CO2 molecule is elongated from 0.07 Å
with respect to the free adsorbate.
Likewise, H2S and SO2 are also chemisorbed onto FeG. In the case of the FeG-H2S
system, H2S binds to FeG in a top configuration through Fe-S bonding since the sulfur
atom has a single lone pair in the H2S molecule. The FeG-H2S system shows an adsorption
energy of 1.19 eV, with a Fe-S bond length of dFe-S=2.31 Å; the interaction almost does not
affect the interatomic distances in the H2S molecule. This adsorption configuration is
10
similar as found by Zhang and co-workers with Eads1.02 eV from PBE calculations
without dispersion corrections[33]; they also proposed FeG as a catalytic support for the
H2S dissociation[33]. Finally, the FeG-SO2 system shows an adsorption energy of 1.80 eV,
where the SO2 molecule is chemisorbed in a parallel conformation onto the dopant atom at
Fe-O and Fe-S distances of dFe-O=1.92 and dFe-S=2.47 Å, respectively. In this case, the
interacting S-O bond is elongated in 0.12 Å with respect to the free SO2 molecule.
Fig. 2. Relaxed potential energy profile (in relative energy, Erel) as a function of the FeG-
adsorbate distances (in parenthesis) in the FeG-adsorbate systems.
Fig. 2 shows the relaxed potential energy surface as a function of the intermolecular
FeG-Gas bond distance. It is clear that the adsorption/desorption process occurs without an
energy barrier in all the cases (the only energy barrier is the adsorption energy). This
property is required for the efficient recovery of the adsorption platform after adsorption,
where thermal annealing could be easily implemented to allow the recovery of the material
without its chemical degradation due to the high dopant-graphene binding energy. For
instance, Shimoyama and Baba have reported the efficient recovery (in 84%) of
synthesized P-doped graphene after thiophene adsorption by means of thermal treatment at
11
1073 K without the thermal degradation of the adsorbent[72]; graphene based sensors have
also shown their surface reactivation at temperatures above 470 K[73].
In order to ensure the applicability of FeG for sensing or gas capture at room
temperature, the stability of the FeG-Gas interactions was studied at 300 K by means of ab-
initio molecular dynamics. The propagation of the bond distances was characterized
thought the radial pair distribution function gab(r) (Fig. 3), which determines the
distribution of distances between pairs of atoms along the overall trajectory. Fig. 3a shows
the intermolecular FeGGas distances, where the chemisorption is noted to be retained
along all the trajectory, thus neither diffusion nor desorption over/from FeG occurs. The
intermolecular distances for the FeG-CO and FeG-H2S systems are in a range dFe-
C=1.762.0 Å and dFe-S=2.122.54 Å, respectively, which are consistent with those
obtained in the ground state (1.88 Å and 2.31 Å for CO and H2S, respectively). While the
bidentated interaction of the FeG-CO2 ground state is retained under dynamic conditions,
the bidentated FeG-SO2 interaction turns into a monodentated after 1.3 fs. For FeG-CO2 the
intermolecular distances are in the range of dFe-O=1.83-2.03 and dFe-C=1.84-2.38 Å. For the
FeG-SO2 system, the range of distance was found to be of dFe-O1=1.72-2.03 Å (similar to
the 1.92 Å in the ground state); since the sulfur atom breaks the bond with the dopant after
1.3 ps, two main distributions of distances were obtained: dFe-S=2.16-2.56 Å and dFe-S=2.94-
3.20 Å. Otherwise, Fig. 3b contains the intramolecular bond distances of the adsorbed
gases, which are consistent with those obtained in the respective ground state and no
chemical transformations are observed. It is worth noting the S-H bond distances in the
FeG-H2S system are quite similar along the trajectory as expected since the hydrogen atoms
do not interact with the Fe atom. Finally, Fig. 3c depicts the intramolecular Fe-C distances
12
in FeG, which are retained in the range of dFe-S=1.70-2.00 Å, ensuring the stability of the
doped nanoadsorbent under gas adsorption at 300 K proposed adsorbant. These results
indicate that neither the absorbent sheet nor the pollutant suffer large structural deformation
that would prevent the usage of FeG surfaces for sensing or gas capture.
Fig. 3. Radial pair distribution function [gab(r)] of: (a) Intermolecular Fe-Gas distances, (b)
intramolecular distances in the gas molecules, and (c) intramolecular Fe-C distance in FeG.
20000 conformations per system were used for statistics.
13
In summary, our results indicate that FeG performs as a promising adsorbent
platform towards CO, CO2, H2S and SO2 molecules, with a best adsorption strength
compared to intrinsic graphene. In addition, the adsorption performance of FeG appears to
be good in comparison with other metal doped graphene surfaces. For comparison
purposes, Table 2 shows the adsorption energies for the interaction of gas molecules onto
another metal-doped graphene; these adsorption energies were obtained by different DFT
functionals. In this regard, the adsorption energy reached by CO and CO2 onto FeG is
considered good by comparison with those reached onto another metal-doped graphene,
which reach adsorption energies of 0.4-1.4 eV (CO) and 0.1-0.2 eV (CO2) (Eads values at
the PBE level of theory). For instance, the adsorption stability of CO onto FeG is
comparable to that reached onto CuG (1.30 eV) and AuG (1.37 eV) with the PBE
functional. In the case of H2S, its adsorption stability onto FeG is 0.2-0.5 eV higher than
onto PtG, SiG and CaG (at the PBE level of theory). However, compared to Co, Sn and Ti-
doped graphene the adsorption strength of H2S onto FeG is slightly weaker. In the case of
the SO2 adsorption, the adsorption stability is enhanced in at least 0.7 eV onto FeG with
respect to CoG and PtG. Finally, Due to the strong interaction of SO2, CO, and H2S, FeG
could catalyze or activate these gas molecules, suggesting the possibility of FeG as a
catalyst substrate.
14
Table 2. Comparison of adsorption energies (in eV) for the adsorption of CO, CO2, H2S
and SO2 onto metal doped graphene and computed at the DFT level of theory.
System CO CO2 H2S SO2
SiG 0.58(LDA)[74]
0.17(PBE)[48]
0.06(PBE)[48] 0.94(LDA)[75]
FeG 1.46(B3LYP)[34]
1.38(PW91)[38]
1.92(LDA)[75]
CoG 0.94(B3LYP)[34]
0.62(PBE)[47]
1.80(LDA)[75] 1.07(PBE)[47]
CaG 0.66(LDA)[75]
SnG 1.43(PBE)[49] 1.78(PBE)[49]
TiG 0.45(PBE)[50] 2.35(PBE)[49]
2.49(PBE-D)[51]
3.20(PBE)[50]
CuG 1.30(PBE)[29]
1.71(PWC)[76]
0.22(M06-L)[77]
AgG 1.01(PBE)[29]
AuG 1.37(PBE)[29]
PtG 1.29(B3LYP)[34] 0.09(PBE)[52] 1.02(PBE)[33] 0.85(B3LYP)[78]
1.06(PBE)[52]
PdG 1.05(LDA)[31]
0.92(B3LYP)[34]
RuG 1.22(B3LYP)[34]
NiG 1.02(B3LYP)[34]
3.2 Bonding nature
The NBO analysis of the donor-acceptor interactions was implemented to
characterize the nature of the FeG-Gas bonding (Fig. 4). It is important noting FeG can
behave as a Lewis acid or base, according to the different adsorbates. In this regard, Fe
atom ([Ar]4s23d
6) develops a sd
2 hybridization to bind with the carbon atoms in graphene
with one monovacancy[25, 63]. After FeG is formed, Fe atom retains two high occupied 3d
orbitals able to interact with the acceptor orbitals of the adsorbates (behaving as a Lewis
base); and, there are three *Fe-C and one *Fe-C antibonding orbitals, which are able to
interact with donor orbitals in the adsorbates (behaving as a Lewis acid)[25, 63].
15
Fig. 4. Natural bond orbitals (NBOs) associated with the donor-acceptor (bonding-
antibonding) interactions in the different adsorption configurations of the FeG-Gas
systems; isosurface values of 0.15 a.u. Arrows indicate the donoracceptor direction.
Color code: White (H), grey (C); red (O); yellow (S), orange (Fe).
We found that the FeG-Gas interaction is mainly dominated by a Lewis-acid-base
mechanism. First of all, FeG behaves as a Lewis acid in the FeG-CO system, where the
high occupied lone pair of the carbon atom of CO (with electron occupation occ1.6e) acts
as donor for all the antibonding *Fe-C and *Fe-C orbitals in FeG (Fig. 4). In this case,
the *Fe-C orbital is highly occupied (occ0.6e) compared to the unoccupied *Fe-C
orbitals, causing a steric repulsion that limits the interaction strength. With respect to the
FeG-CO2 system, it is necessary pointing-out that the CO2 molecule has a linear molecular
geometry (O-C-O=180°) in its isolated state due to the two C-O bonds according to its
more stable Lewis structure. When CO2 interacts with FeG, the O-C-O angle decreases to
154°, resulting in a destabilization of the CO2 structure; in consequence, C-O bonds break,
16
and the carbon atom results with two lone vacant 2p orbitals with single occupancies
(occ0.9e). As noted in Fig. 4, the 3dz2 orbital of Fe (occ=1.7e) acts as donor for one
carbon lone vacant 2p orbital in CO2, which must result in a lower stability compared to the
FeG-CO system. Additionally, in a back-bonding process, one oxygen lone pair in CO2
(occ=1.6e) acts as a donor for the *Fe-C orbital in FeG (occ=0.6e), increasing the binding
instability due to steric repulsive interactions, and decreasing the adsorption strength with
respect to the FeG-CO system. The latter also increases the C-O bond in 0.07 Å.
Finally, we analyze the sulfur containing gases and their interactions with FeG. In
the case of the free H2S molecule, sulfur atom has a non-bonding 2p lone pair (occ1.7e);
when H2S is chemisorbed, the 2p lone pair delocalizes toward one *Fe-C bond of FeG,
but the interaction strength is decreased when the same sulfur lone pair acts as a donor for
the *Fe-C bond in the adsorbent. Otherwise, like in the CO2 case, the FeG-SO2 interaction
breaks the S-O bonds in the SO2 molecule, which results in a single occupied 2pz orbital
for the S atom. As noted in the Fig. 4, the sulfur 2pz orbital mixes with the iron 3dz2 lone
pair orbital in FeG, which is doubly occupied as noted above. The latter results in a strong
Fe-S bond (occ=1.5e), but also in a relatively high occupied *Fe-S bond (occ=0.8e).
Consequently, the oxygen 2p lone pair orbitals of SO2 act as donors for the *Fe-S orbital,
establishing a three-center bond.
3.3 Electronic properties
To get more insights into the FeG-Gas interactions, relevant electronic properties
were analyzed such as the charge transfer, electron density difference, eigenvalues of
frontier orbitals, and density of states (DOS). In the first place, the charge in the adsorbed
17
molecule (qAD) is used as a measure of the charge transfer; this is a positive value of
indicates electron transfer in the FeGdirection. In this regard, CO2 and SO2 act as
acceptor molecules, gaining electrons and forming 0.02 and 0.12 holes per molecule in
FeG, respectively. Conversely, CO and H2S lose electrons, introducing 0.13 and 0.26
electrons per molecule in FeG. In this regard, Song and co-workers have experimentally
reported that chemisorbed H2S transfers electrons to synthesized SnO2/graphene based
sensors, resulting in an improved sensing response by decreasing the electrical
resistance[79]. Zhang and coworkers have also reported the donor character of H2S
towards FeG through PBE calculations[33, 80]. Therefore, the charge transfer is expected
to induce the larger changes in the FeG conductivity as a result of the charge doping[27].
Note that in most of the cases the amount of charge transfer is underestimated by
theoretical calculations because of this value depends on the number of atoms in the
graphene layer[17, 81], where graphene models containing more than 1000 carbon atoms
appear to be reliable to obtain accurate values of electron transfer[17, 81]. The analysis of
charge transfer is in agreement with the plot the electron density difference (r) (Fig. 4).
In the case of FeG-CO2 (Fig. 5b) and FeG-CO2 systems (Fig. 5d), electron density
accumulates in the 2p orbitals of oxygen and sulfur atoms of the adsorbates. Conversely, in
the case of FeG-CO (Fig. 5a) and FeG-H2S systems (Fig. 5c), the transferred electron
density is mainly retained in the chemical bond without significant polarization, and the 2p
lone pairs of the adsorbate suffer an electron depletion; while, the accumulation of electron
density is noted in the carbon atoms surrounding the dopant in FeG. In all the cases, the
dopant atom suffers outflow of electron density under molecular adsorption; the analysis of
atomic charges shows that Fe atom loses 0.3|e| under gas adsorption.
18
Table 3. Mulliken charge of the adsorbate after adsorption (in |e|); eigenvalues of the
HOMO and LUMO levels (HOMO and LUMO), and the HOMO-LUMO energy gap (HL) of
the FeG-adsorbate systems compared to their isolated fragments. Energies are in eV.
system qAD HOMO LUMO HL
FeG -4.27 -3.73 0.55
CO -8.79 -1.88 6.91
CO2 -8.77 -0.23 8.54
H2S -6.16 -0.43 5.73
SO2 -7.58 -4.40 3.18
FeG-CO 0.13 -4.49 -3.40 1.09
FeG-CO2 -0.02 -4.47 -3.49 0.99
FeG-H2S 0.26 -4.25 -3.24 1.02
FeG-SO2 -0.12 -4.52 -3.95 0.57
Fig. 5. Electron density difference [(r)] obtained by adsorption of CO (a), CO2 (b), H2S
(c) and SO2 (d) onto FeG. (r)=AB(r)-A(r)-B(r), where AB(r) is the electron density of
the AB system (adsorbate-adsorbent), and A(r) and B(r) are the electron density of each
fragment. Outflow and accumulation of electron density are displayed with sky-blue and
yellow colors, respectively. Isosurface value of 0.003 e/Bohr3.
19
For a better understanding of the electronic properties of the FeG-Gas, the frontier
molecular orbitals (HOMO and LUMO) were analyzed (Table 3). Due to the weak and
long-range interaction of the gas molecules onto intrinsic graphene, a negligible effect onto
the conductance properties of graphene is expected under gas adsorption. However, FeG
behaves as a semiconductor or a semimetallic material with respect to graphene[82], and its
strong chemical interaction with gas molecules could produce additional changes in its
electronic structure as noted from the charge transfer analysis. Indeed, the HOMO-LUMO
energy gap values (HL) of the FeG-CO, FeG-CO2 and FeG-H2S are increased in at least
0.54 eV with respect to the free adsorbent as observed in Table 3. Because of the HOMO
level of the gas molecules is far away from the HOMO level of FeG (-4.27 eV), the
increase in the HL parameter is mainly governed by destabilization of the LUMO level in
up to 0.5 eV with respect to the free adsorbent. In this regard, the partial DOS plots of
these systems (Fig. 6) clearly show that the frontier energy levels are mainly affected by
hybridization of the unoccupied orbitals of the gas molecules in the conduction band of
FeG. Taking into account that the bandgap value is directly proportional to the conductance
(HL/kT, where k is the Boltzmann constant and T the temperature), these results suggest
that FeG is a promising sensor material for gas sensing because of its conductance
decreases under gas adsorption. On the other hand, the FeG-SO2 system shows a slight
increase in the HL value with respect to the isolated FeG, which is only of 0.02 eV. The
latter is due to that the unoccupied 2* orbital of SO2 (at -4.40 eV) is lower in energy than
the HOMO of FeG (at -4.27 eV); then, the LUMO of the FeG-SO2 system is mainly a 2*
orbital with a low hybridization with the occupied 3d states coming from FeG. Therefore,
these results indicate that the electronic structure of FeG is slightly sensitive to the
20
adsorption of SO2, and it could be considered as an inefficient material for SO2 sensing.
However, it is important to note that molecular dynamic trajectories showed that the
adsorption configuration in the ground state of the FeG-SO2 system is changed at room
temperature; thus, its three-center bond is displaced towards a single bond interaction. In
this adsorption conformation, we observed that the LUMO of SO2 is extra stabilized and
hybridizes with the HOMO orbital of FeG due to 3d sates; the latter causes a decrease in
the HL value of 0.3 eV. This is in agreement with the SO2 adsorption onto Ti and Al-
doped graphene, where the SO2 mixes with the 3d sates of the metal dopant near to the
Fermi level of modified graphene[50, 83]; as well as the bandgap of Pt-doped graphene is
also decreased under SO2 adsorption[78]. Consequently, the adsorption of SO2 onto FeG
could be recognized by an increase in the conductance of FeG at room temperature.
21
Fig. 6. Partial density of states (DOS) plots of the FeG-Gas systems. Blue line corresponds
to the states of Fe in FeG; red line corresponds to the states of gas molecule. The vertical
green line indicates the position of the HOMO level.
3.3. Adsorption in O2 environments
Although gas pollutants can be present in oxygen-free environments as those in
industrial waste gases, the influence of O2 molecules during the capture of gas molecules
onto FeG could take into account when an aerobic environment is considered. In this
regard, O2 reaches an adsorption energy of 1.6 eV when adsorbed onto FeG[25, 38]; our
22
computations show a value of 1.68 eV, which is near or high that the adsorption of CO,
CO2, H2S and SO2. Therefore, could have a probability of poisoning of the adsorption
platform in oxygen environments, avoiding the sensing and adsorption properties of FeG.
However, it necessary noting that resistive graphene sensors have remained with high
selectivity and sensitivity for gas sensing even in a background of N2 and O2 gases[84].
Considering that O2 is a zero dipole moment molecule (without charge polarization), it is
reasonable to expect adsorption of polarized molecules such as CO, H2S and SO2 (dipole
moments of 0.32, 1.35 and 1.70 Debye) will be favored in a first step by charge-controlled
interactions. Even, CO2 breaks its linear geometry to an angular conformation when
approaches to FeG, showing a dipole moment of 0.61 eV. These statements support that
polarized molecules are preferably adsorbed in a first step rather than O2, even when
differences in its adsorption energies emerge. Despite the latter, experimental studies of
graphene sensors for formaldehyde detection (and based on ZnO and In2O3 substrates)
show that adsorption and detection take place in O2 environments by co-adsorption
mechanisms[85-88]. This is the sensing properties remain by detection of different
intermediate compounds that are formed in a co-adsorption regime[85-88]. To get further
insights into this last point, the co-adsorption of gas molecules onto FeG in the presence of
one O2 molecule (in its triplet state) as an approach was explored; in other words, the most
stable adsorption configurations for the gas-O2-FeG systems were computed, where the gas
molecule and O2 are adsorbed on the same dopant site. The co-adsorption configurations
are displayed in Fig. 7, and their properties are in Table 4.
23
Fig. 7. Co-adsorption configurations of CO, CO2, SO2 and H2S onto FeG in the presence of
O2. Distances are in angstroms (Å) and angles in degrees (°). Color code: White (H); grey
(C); red (O); yellow (S); orange (Fe).
Table 4. Properties of the co-adsorption configurations: Adsorption energies (Eads), charge
of the adsorbate (qAD) and O2 molecule (qO2), and HOMO-LUMO energy gap in and -
channels (HL).
system Eads Eads-af qAD qO2 HL- HL-
FeG-O2-CO 2.11 0.41 0.20 -0.18 0.85 0.42
FeG-O2-CO2 1.34 -0.35 0.09 -0.21 0.93 0.39
FeG-O2-H2S 1.87 0.18 0.18 -0.31 0.83 0.63
FeG-O2-SO2 1.96 0.27 -0.15 -0.14 0.80 0.47
FeG-O2 1.69 - - -0.34 0.87 0.66
Assuming the interaction with one O2 molecule, Fig. 7 shows that the gas molecules
bind to FeG in presence of O2 in a similar way as obtained in the free oxygen states. For
comparison purposes, we obtained that O2 is adsorbed in a parallel configuration (side-on)
onto isolated FeG, without dissociation and adsorption energy of 1.69 eV, which is in
agreement with previous reports[38]. Besides, hybridization between the occupied 3d
orbitals of Fe with the 2* orbitals of O2 causes the charge transfer in the FeGO2
24
direction (qO2=-0.34|e|), which elongates the OO bond in 0.14 Å due to the occupation of
its antibonding 2* orbitals. The strong interaction is characterized by a strong coupling of
the -orbitals of FeG and O2 near to the HOMO and LUMO levels of the FeGO2 system
as noted in the partial DOS plot (Fig. 8). The latter because the unoccupied -orbitals of O2
are available as acceptors, this is the single occupied 2* orbitals.
In a co-adsorption regime, O2 is mainly adsorbed in an O-end configuration
(binding through a single Fe-O bond), excepting for H2S. CO and SO2 are co-adsorbed with
similar bond lengths as in the free O2 case; while, the intermolecular bond length is
increased in 0.4 Å for CO2 and H2S in the presence of O2, suggesting the decrease in the
adsorption strength. Indeed, the adsorption strength of the gas adsorption is sorted as
CO>SO2>H2S>CO2 as determined from the adsorption energies in Table 4, which range
from 2.11 to 1.34 eV. Otherwise, it was considered the adsorption energy of gas molecules
onto FeG but after O2 uptake, where the adsorption energy (Eads-af) is obtained as:
Eads-af=EadsEads(FeG-O2) (2)
where Eads is the adsorption energy of the whole FeGO2Gas system, and Eads(FeG-O2) is the
adsorption energy of O2 onto FeG. In this framework, the gas adsorption after the O2 uptake
is favorable only for CO, H2S and SO2 with Eads-af values of 0.41, 0.18 and 0.27 eV.
Conversely, the CO2 co-adsorption is an unstable process after the O2 binding in the
adsorption site. The latter is explained because of some of preparation energy is required
for the change between side-on and end-on configurations of the adsorbed O2. Therefore,
the CO2 adsorption is expected to be unstable in an oxygen environment. Additionally, note
that co-adsorption of CO2 is almost 0.8 eV less stable than co-adsorption of CO, while the
25
co-adsorption of H2S and SO2 is almost similar. These results suggest that FeG is a
promising candidate for CO oxidation in the presence of CO/O2 mixtures.
With respect to the electronic properties, O2 acts as an acceptor molecule onto FeG,
introducing 0.34 holes/molecule into the adsorbent. The latter causes the decrease in the
conductance of FeG, which is characterized by an increase of the HOMO-LUMO energy
gap; the HL index reaches values of 0.87 and 0.66 eV in the alpha and beta channels,
respectively (keep in mind that O2 causes the spin polarization of the system). Under co-
adsorption of CO, CO2 and SO2, the qO2 parameter (Table 4) indicates that O2 remains as an
acceptor molecule, but the amount of introduced holes in FeG is decreased, this is the
amount of transferred electrons to the 2* orbitals of O2 is decreased. The latter must result
in an increased conductance with respect to the FeG-O2 system as a result of the decrease in
the strong hybridization between the FeG and O2 states near to HOMO and LUMO level in
the -channel. Indeed, the HL values of the beta channels decrease in up to 0.27 eV with
respect to the FeG-O2 system as a result of the stabilization of the LUMO level. From the
DOS plots (Fig. 8), it is observed that the co-adsorbed gas molecules change the way as O2
hybridizes with FeG in the -channel of the conduction band due to the change from the
end-on to the side-on configuration. In this way, the amount of transferred electrons to the
2* orbitals of O2 is decreased and the LUMO level is stabilized. Conversely, the amount
of transferred electrons to O2 is not decreased by the adsorption of H2S, mainly due to the
low acidic character of H2S compared to CO, CO2 and SO2. Additionally, the larger Fe-S
bond distance in the co-adsorption configuration (2.68 Å) does not favor the complete
change from the end-on to the side-on configuration of O2. The latter results in a low
response of the bandgap of and -channels. Indeed, the partial DOS plot of the FeG-O2-
26
H2S system shows that H2S almost does not change the electronic structure compared to the
FeGO2 system, neither - nor -channels. Therefore, the sensing response remains high
for CO, CO2 and SO2, but the weak interaction of CO2 with FeG in O2 environments could
be difficult to reach an chemisorption state; in addition, the sensing response of FeG
towards H2S is suppressed in O2 environments.
Fig. 8. Partial density of states (DOS) plots of the FeG-Gas systems. Blue line corresponds
to the states of Fe in FeG; red line corresponds to the states of the adsorbed gas molecule;
orange line corresponds to the states of O2. The vertical dotted line indicates the position of
the HOMO level.
27
4. Conclusions
The implementation of Fe-modified graphene nanosheet as a nanoadsorbent towards
harmful gas pollutants (CO, CO2, SO2 and H2S) was characterized through a detailed DFT
study. It was found that these gas pollutants are chemisorbed onto FeG with adsorption
energies in the range of 0.54 to 1.80 eV, improving the adsorption strength in at least 390%
compared to those onto intrinsic graphene (on the range of 0.08 to 0.28 eV). Analyses of
the chemical binding indicated that the stability of the FeG-Gas interactions is dominated
by Lewis-acid-base interactions; the chemisorption remain strong at room temperature (300
K) as determined from molecular dynamics trajectories. The acceptor/donor character of
CO, CO2, SO2 and H2S molecules when adsorbed onto FeG causes charge transfer
processes that are responsible for the change in conductance of FeG; thus, the response of
the HOMO-LUMO gap of the FeG system under gas adsorption is expected to be useful for
sensing applications. On the other hand, it was also explored the effect of O2 molecules (co-
adsorption) on the adsorption process of CO, CO2, SO2 and H2S. In these cases, it was
found that the CO2 adsorption turns unstable in the presence of O2; while, the response of
the electronic properties of FeG towards H2S is suppressed in the presence of O2.
Therefore, these results give new insights into the emerging new applications of Fe-doped
graphene in gas capture/filtration devices or solid-state gas sensors.
Acknowledgments
Powered@NLHPC: This research was partially supported by the supercomputing
infrastructure of the NLHPC (ECM-02). N.V-E and D.E.O acknowledge the Ph.D.
fellowship from CONICYT.
28
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